dexter og

I made a world for my friend and me, and we've eventually realized that we want to have oceans in our world, but I accidentally made the landcover 100% in the creation. I am aware you can change this with /worldconfig in-game, but it doesn't seem to have an effect on worlds with 100% landcover. Is there any way to rectify this mistake? Preferably, I'd like a world with 70% landcover and 200% landcover scale.